Lunar Laser Rangin

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Lunar Laser Ranging
I did my presentation in class on radar guns and the science behind them. With radar guns the most common but older technology uses radio waves and the principles of Doppler shifts to calculate the speed of an object. The newer, more precise technology is called LIDAR (Light Detection and Ranging) technology that uses laser pulses and the speed of the light to calculate speeds. To give a quick summary: the old radar guns measure the change in frequency of a radio wave bounced off a moving object while the LIDAR systems measure the change in time it takes two light pulses of lasers to hit a moving object and return. This laser technology is now being used to track the moon’s orbit, measure gravitational forces, and challenge Einstein’s theory of relativity. Lunar laser ranging is the process of “pinging” laser pulses off reflectors placed on the moon and measuring the time it takes for the lasers to return. Since we know the speed of light we can accurately measure the distance the light traveled. On average the time it takes for light to make the roundtrip is between 2.34 to 2.71 seconds depending on how far away the moon is at that moment. The distance ranges from 351,000 km to 406,000 km. This experiment is possible because in 1969 the Apollo 11 astronauts placed retroreflector arrays on the lunar surface. Following this first mission the Apollo 14 and 15 missions followed suit and placed corner cube retroreflector arrays on the lunar surface. The three Apollo reflectors in combination with a French built reflector left by the Soviets allow scientists to accurately map the moon as it orbits and rotates. These reflectors require no power and have been operating normally since they were installed nearly forty-three years ago. Scientists make measurements using each of the four available reflectors over the course of a half-hour period. This is repeated every few nights giving us a wealth of data to use in exploring the effects/behavior of gravity. You may ask why these reflectors are even necessary since we already know light can be reflected off the moon (we see it reflecting sun light). There are some technical challenges that scientists face when trying to accurately measure laser pulses reflected from the lunar surface. First of all an outgoing laser pulse from a collimating (parallel, non diverging) telescope with a beam divergence of 3-4 seconds of arc spreads to about 2-7 km on the moon’s surface (Dickey). If there were no reflectors on the moon then the laser pulses would be backscattered producing a weak barely detectable signal. This returning light would also have the characteristics of the local lunar topography. This method would not yield accurate nor feasibly collectible data. This is why the retroreflectors were put into place. The design of the reflectors is relatively simple to understand. The corner cube reflectors reflect incident light back to its point of origin. The Apollo 11 and 14 reflectors consist of 100 arrays and the Apollo 15 reflectors consists of 300 3.8 cm corner cube reflectors. These cubes are mounted in an aluminum panel. Corner cube retroreflectors are designed to reflect any ray or beam entering cube that has a prism face back to the source. It will do this regardless of the orientation of the prism. A mirror will only do that at the normal angle of incidence. This means that corner cube retroreflectors are most useful where precision alignment is difficult or extremely impractical i.e. on the moon. They feature three total internal reflections and will function even at very large angles of incidence. However, since the reflectors are relatively small in comparison to the distance the light is traveling only about 10-9 of the impinging light can be collected by the arrays and reflected back to earth. “The angular spread of the returning pulse is set by diffraction, polarization properties, and the irregularities of the arrays’ individual corner cubes which, in the...
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